He served as the agent of the Imam Hadi, Imam Hasan 'Askari, and Imam Qa'im (peace be upon them)." Such an opinion of him was universally held by all other authors of biographical dictionaries. Thus, 'Allama Bihbahani, in addition to praising 'Uthman, says that he was actually accredited by the Imams Hadi and Hasan 'Askari.
Ahmad b. Ishaq relates the incident in which he asked the tenth Imam Hadi regarding the person with whom the Shi'a should deal and whose guidance they should accept as coming from the Imams. The Imam said: "'Uthman b. Sa'id is my trusted agent. If he relates something for you then he is telling the truth. Listen to him and obey him because I trust him." When Imam Hasan 'Askari was asked a similar question he mentioned both 'Uthman and his son Muhammad as his trusted agents. Moreover, he also required his followers to listen to and obey Uthman. These reports were so widespread among the companions of the last Imams that they became the source of the respect and trust with which 'Uthman b. Sa'id was held.
On one occasion Muhammad b. Isma'il and 'Ali b. 'Abd Allah came to Samarra to visit Imam Hasan 'Askari. There was a group of Shi'a visiting the Imam at that time. Suddenly, the servant came and announced that a group of villagers, shabbily dressed, were seeking permission to enter the presence of the Imam. The Imam said: "They are Shi'a from the Yemen." Then he told the servant to ask 'Uthman to be prepared for the visitors. Within a short while 'Uthman was ready. The Imam said to him: "'Uthman, you are our trusted agent. Receive the goods this group has brought." This elevation of 'Uthman, according to the narrators of the report, was done in order to let the Shi'a know the status of 'Uthman. In fact, towards the end of that visit Imam Hasan 'Askari declared to the group saying: "Let it be known to you that 'Uthman b. Sa'id is my agent and his son will be the agent of my son Mahdi."
Imam Hasan 'Askari revealed his son to the group of forty people among his followers, including 'Ali b. Bilal, Ahmad b. Hilal, Muhammad b. Mu'awiya, and Hasan b. Ayyub and said: "This is your Imam and my successor. Obey him! Know that after this time for a while you will not see him. Listen to what 'Uthman b. Sa'id says and follow his instructions because he [Uthman] is the successor of your Imam. The management of the affairs of our people will be in his hands."